Visually not a lot has changed since my last update. In preparation for printing most changes have been minor dimensional tweaks. Generally speaking the clamping interfaces are opened up slightly to ensure they go together easily and actually clamp to the tube. Fastener holes are closed up slightly to allow drilling of a clean hole after printing.
One of the last unanswered questions I had with this design was where the battery connector would join to the power distribution board and how it would generally fit into the layout. My initial plan was to run it out through a grove in the outer surface of one of the end clamps. The reality of this is that there is very little space between tube and the plates on each side, certainly not enough for 16AWG silicone wire (as fitted to the battery) without squashing it. The plan instead is to run the connecting wires out through one of the slots which can now be seen in the top plate above. The battery is raised with the self adhesive rubber ‘feet’ stuck on top. Also now visible in the image above are the cutouts to accommodate a battery strap or two.
I’ve also done a lot of the work getting the top board laid out. My approach with this is more ‘suck it and see’ than design perse. I’ve not checked current carrying capacity of via’s or the +ve and -ve planes. The board is laid out with:
- Direct connections for battery power to the escs
- A place for the 5v step-down
- 5v accessory connection points front, rear and centre
- JST connector for 5v connection to the flight controller
- JST connector for battery voltage to the flight controller.
I’ve been debating how integrated to make the bottom board (hosting flight controller and receiver). I think it will probably be a better choice to keep it simple and mount these components with fasteners or double sided tape rather than trying to solder them in permanently. I am however a bit of a sucker for the clean look of soldering them in directly as impractical as this is for servicing. Stay tuned for an update on this front, I will need to make a decision in short order as I will now be waiting on fabrication of these for assembly.
With another 40 minutes on the clock I have added the final detail to the chassis structure for the 215 Hopper. All that remains now for these parts to be ready for printing is some minor tweaks to be sure everything will fit together.
In order to positively hold the ends of the side pieces I have added a small tab to the top and bottom. These tabs will snap in to slots cut into the top and bottom plates. My plan initially was for the side pieces to key into the centre clamp pieces but in order to achieve this in a robust manner I was concerned I would have to give up to much of the clamping material. Also of note are the tapered faces I have added to the join surface. This change in shape of the seam should help secure the side plates in a longitudinal direction.
When assembled the design makes for quite a tidy little package if I do say so myself.
Laid out flat, this is what a full ‘kit’ of the core printed parts will look like. There will still be other non-critical items such as the antenna tube mounts to add to this print list.
To complete the build of the chassis the following pieces will also be needed:
- Top & Bottom Plates
- 12mm tube (any will work but I’ve got some carbon tube on hand which was purchased for the MultiChase Project)
At the end of last week I found myself at the intersection of 3 lines of thought:
- What could I transfer the flight systems on my Spidex220 to for a more rigid platform.
- A friend has a FDM style 3D printer. I have reservations about the tolerances achievable with it for printing the finer parts of the MultiChase Project (which has always been designed with SLS printing in mind). What can I do to involve him and his printer.
- How quickly I can come up with a viable solution to 1 and 2.
These thoughts have resulted in a speed challenge of sorts for myself. I have decided to come up with a simple quad that I hope will be printable on a basic FDM printer. I’ve not got any hands on experience with the equipment but I figure if I keep the design simple and the tolerances forgiving then I can’t go too far wrong. I don’t want to divert too much time away from the MultiChase Project hence the speed challenge.
After 3 hours work spread over 3 sessions I have come up with the framework for the solution. The motor spacing is slightly smaller than the Spidex220 I will be cannibalizing. Motor spacing is 215mm as hinted at by the name I have given it, 215 Hopper.
The design is heavily inspired by the Flite Test VersaCopter. The layout is basically identical only it is much more compact and uses 3D printed parts rather than laser cut plates. Hopefully with more material supporting the tubes durability will be better.
All of the flight electronics, including the ESC’s are carried inside the chassis. ESC’s will be attached to the top panel (and power distribution board), whilst the flight controller, 5v regulator and receiver will be on the bottom panel.
The ESC’s I have been using in the Spidex220 have not inspired a lot of confidence, often exhibiting inconsistent performance. As such it took very little for me to decide to try something else. When they didn’t quite fit the width I was targeting they were dropped in favour of a different design, the ZTW Spider Series 18A Opto Lite‘s. The lack of a BEC on this ESC has necessitated the inclusion of a 5v regulator to power the flight controller and receiver.
In isolation the 215 Hopper will look like this.
With the battery, a 1400mAh Multistar LiPo (3S shown but I might get some 4S packs if everything works out), and a GoPro Session on board a real sense of scale hits me in the face. Its very compact! As hinted at by the presence of the GoPro I hope to add FPV hardware in the future however I am not designing with that as a requirement.
There is still some detail to work out on the side plates and I need to design some clip in antenna tube mounts. Once that is sorted I will make another post showing the assembly detail and it will then be ready for printing. On the time front, the quoted 3 hours is exclusively CAD time. I have spent probably that much again looking for and purchasing various bits of hardware to make this happen but I am quite pleased with progress.
Having reached a stage where I am happy with the general structure of the two core chassis pieces I thought it time for an update. Shown here is the lower half of the structure with the servo module nestled in the middle.
I have not yet added the belt tensioning mechanism but the intention is for a screw to be inserted through the front face of the chassis to push on the servo carrier.
This is how the lower chassis piece appears in isolation. There is still plenty of refinement to follow including weight reduction (only where significant gains can be made given that this is primarily a proof of concept) and rounding of sharp corners. The upper half is a direct copy of this piece for the bearing supports and servo mounting but the rear end changes to make space for the flight controller.
Whilst the flight controller was on my mind a made a point of checking pin locations on the intended board (The RMRC Seriously Dodo Flight Controller). My plan of attack is to mount the board with the USB connector facing rearward and then install 90° pins backwards on to the outputs, that is to say so that the connectors and wires will run across the board rather than away from it as would usually be the case.
I have also attended to the positioning of the receiver I will be using, the FrSky X4R (my RX of choice due to its S.Bus and SmartPort interfaces). As shown here it will be on the front of the chassis (behind the nose cone).
Whilst working on the chassis pieces I also added mounts for the antenna. By no means are they optimised for diversity, I am not expecting reception issues given I will only be flying this with direct line of sight.
There are still a lot of fine details to attend to but hopefully with some time away from work during the holiday period I will be able to get all the 3D printed parts buttoned down and out for printing (no printer in house unfortunately).
Over a couple of hours today I have worked out the CAD details for the timing belts and pulleys. Now teeth and proper pitch lines are modeled in to all parts. I am not 100% sure that the geometry of the teeth on the pulleys is correct but it should be 3D printable and work at a basic level.
The belt is a 48 tooth T2.5 timing belt (already in the post from an aliexpress supplier) and the pulleys are both 20 teeth. There is of scope for increasing size of the pulley on the arms for a reduction drive but this will likely require a longer belt.
If the 3D printed pulleys work out then in the future I will put together a guide for modeling the teeth in a CAD program as I could not readily find advice of this nature.
Over the last few days I have put several hours into development of the main chassis structures. Between the two main parts several needs have to be met including:
- Supporting the front arm bearings and the associated loading
- Mounting the flight controller
- Supporting the servos in an adjustable manner (for belt tension)
- Fitting between the power distribution plates and around all the moving components
- Attaching the rear boom
So far as seen in the image below the external shape is mostly in place. Internally the bearing support area is mostly sorted out but the servo mounting is not. As such I will hold on images of internal detail until that is in a ‘working’ state.
Also visible in the image is the rear boom clamping. A slotted hole penetrates the rear of the bottom chassis structure which is then clamped through the two holes in the side. Currently the supporting structure around this area is very deep so the clamping may not be particularly effective (i.e. structure may be to stiff), I will need to reassess.
There is further external detail to work out around the flight controller mounting and battery. You may also note that I have changed the propellers. Crucially to a 5″ diameter but also shown here with 3 blades. The data I have seen suggests that a 5×4 propellor provides similar thrust to a 6×3 propellor (at the expense of some efficiency) and my experience with 5×4 props on the Spidex 220 has made me happy to consider this an alternative. I have not yet reduced the frame size so a 6″ propellor is still an option.
Also added is a front nose cone, primarily for aesthetic reasons at this point but it will surely improve the aerodynamics somewhat too. It is currently only a hollow place holder. The intention is to make it a snap fit on to the front of the chassis structures. That will be another learning experience and experiment in 3D printing to go along with everything else experimental on this airframe.
To facilitate the adjustment of the belt tension I have added a slider carriage of sorts to the servos which will fit into slots on the chassis structures. I’ve yet to add the tooth detail to the pulleys for 3D printing.
The final little detail to show off today is a holder for an XT60 socket. I decided that having the connector flat will mean less wire hanging low off the bottom of the craft.
After looking at the range of motion that was going to be achievable with the previously shown boom control arms I was dissatisfied. Only about 70° was realistic. As a rough goal I would like to achieve 135° (90° forward and 45° backward). To get this range of motion and maintain a compact envelope I have decided to investigate the use of a belt drive system. Shown below is the layout I have come up with. The servos are laid on their side and stacked on top of each other and custom pulleys are used at both ends. It may be possible to source a pulley to fit the servo directly but at this stage I have not come across a source so I intend to pursue a 3D printed pulley attached to a standard round horn.
The power distribution philosophy has also seen some development. Primarily I have split the ESCs onto separate top and bottom boards. These boards will also serve as sandwich plates on the outsides of the internal structure (supporting bearings, servos etc.). I’ve also decided to forgo the use of board mounted bullet connectors and instead solder the motor wires either directly to the ESCs or onto the power distribution boards. I learnt from the construction of the Spidex 220 that this is really not as much of a complication as I had imagined it might be. The extra board real estate also opens up a good spot for the XT60 connector, the battery wire would loop around and in on itself to connect.
All together this creates a fairly tall stack height. As I develop the conceot further I hope to find ways to minimise it.